1. Field of the Invention
This invention relates generally to a tunneling-tip based sensor, and especially to a sensor for detecting changes in the physical properties of materials subjected to various environments.
2. Description of the Prior Art
Sensors may be used to detect conditions in the environment or to determine the effect of these conditions on a particular material. For example, sensors are currently used to detect, inter alia, acoustic fields, electrical fields, magnetic fields and temperature, as well as the effects of changes in these conditions upon the physical characteristics of various materials. Generally, a compact sensor having high sensitivity is preferred.
Presently, the most sensitive instruments for measuring weak magnetic fields are the superconducting quantum interference devices (SQUIDs) which are capable of detecting fields as small as 10.sup.-11 to 10.sup.-12 gauss. SQUIDs, however, must be operated at cryogenic temperatures which may limit their general utility.
Fiber optic magnetic sensors can also measure small magnetic fields. Two basic approaches for magnetic field detection with a fiber optic sensor exist: Faraday rotation and the magnetostrictive jacket or stretcher approach. In the former method, paramagnetic impurities in the fiber optic material cause a rotation of the polarization plane of light transmitted through the fiber; this change in polarization is used to detect magnetic field changes. In the latter method, which is much more sensitive, a magnetostrictive coating is applied around the fiber. Changes in the length of the fiber upon exposure to magnetic fields cause interference effects in the light being propagated through the fiber optic material. Sensitivities of about (10.sup.-5) to (10.sup.-8) gauss have been measured with a fiber length of one meter.
Resonance magnetometers, especially the optically pumped alkali vapor magnetometers, also have sensitivities approaching (10.sup.-10) gauss. In addition, there are many other types of magnetometers based on magnetic induction. These include the vibrating and rotating sample magnetometers, the vibrating coil magnetometer, and the well known fluxgate magnetometer. These magnetometers generally have sensitivities less than or equal to 10.sup.-6 gauss. The fluxgate magnetometer deserves special mention because it is one of the oldest methods and perhaps most used due to its reliability, relative simplicity, stability, economy, and ruggedness.
The more sophisticated devices that have high sensitivity such as the SQUID and the resonance magnetometers have some operational shortcomings. For example, the SQUID magnetometer must be operated at cryogenic (or liquid helium) temperatures. Liquid helium is expensive and has a limited lifetime (hours) before the dewar needs to be refilled. The optically pumped alkali vapor resonance magnetometers can be quite large and require a laser system to operate them. The development of a compact, sensitive, room temperature magnetic field detector would be advantageous to the magnetic field sensing community.